The present invention relates to a radiological imaging apparatus, and more particularly to a radiological imaging apparatus ideally applicable to X-ray computed tomography, positron emission computed tomography (hereinafter referred to as “PET”), single-photon emission computed tomography (hereinafter referred to as “SPECT”), digital X-ray examination flat panel detector, and similar equipment.
Radiological imaging is a non-invasive imaging technology to examine physical functions and conformation of a medical examinee as a subject. Typical radiological imaging devices are X-ray computed tomography, digital X-ray examination, PET, and SPECT devices.
PET is a method for administering a radiopharmaceutical (hereinafter referred to as a “PET pharmaceutical”) containing positron-emitting nuclides (15O, 13N, 11C, 18F, etc.), which are radionuclides, to a medical examinee, and examining locations in the examinee's body where the PET pharmaceutical is heavily consumed. More specifically, the PET method is used to detect γ-rays that are emitted from the medical examinee's body due to the administered PET pharmaceutical. A positron emitted from the radionuclides contained in the PET pharmaceutical couples with an electron of a neighboring cell (cancer cell) to disappear, emitting a pair of γ-rays (paired γ-rays) having an energy of 511 keV. These γ-rays are emitted in directions opposite to each other (180°±0.6°). Detecting this pair of γ-rays using radiation detectors makes it possible to locate the two radiation detectors between which positrons are emitted. Detecting many of these γ-ray pairs makes it possible to identify the locations where the PET pharmaceutical is heavily consumed. For example, when a PET pharmaceutical produced by combining positron-emitting nuclides with glucose is used, it is possible to locate carcinomatous lesions having hyperactive glucose metabolism. The data obtained is converted to individual voxel data by the filtered back projection method, which is described on pages 228 and 229 of IEEE Transaction on Nuclear Science, Vol. 21. The half-life period of positron-emitting nuclides (15O, 13 N, 11C, 18F, etc.) used for PET examination ranges from 2 to 110 minutes.
In PET examination, γ-rays generated upon positron annihilation attenuate within the human body so that transmission data is imaged to compensate for γ-ray attenuation within the human body. Transmission data imaging is a method of measuring the γ-ray attenuation within the medical examinee's body by, for instance, allowing γ-rays to enter the examinee's body using cesium as a radiation source and measuring the radiation intensity prevailing after penetration through the examinee's body. The PET image accuracy can be enhanced by estimating the γ-ray attenuation within the examinee's body from the measured γ-ray attenuation rate and correcting the data derived from PET examination.
A method for increasing the PET examination accuracy is described on page 15 of Medical Imaging Technology, Vol. 18-1. This method is used to insert a reflection plate into a crystal, acquire the information about depth with a DOI (Depth-Of-Interaction) detector, and reconstruct the image according to the acquired information to improve the image quality. For the use of this method, it is necessary to use a radiation detector that is capable of acquiring the information about radiation detector's position in the direction of the depth.
However, the use of a DOI detector involves image deterioration, which is caused by a decrease in the amount of signal transmission substance. When, for instance, a 5 mm square BGO scintillator is used, approximately 200 photons are generated to function as a signal transmission substance when there is a 511 keV incident γ-ray. However, when photons are partly reflected by a reflection plate as in the use of the DOI detector noted above, the amount of signal transmission substance decreases. When the quantity of signal transmission substance reaching a photomultiplier tube is N and the incident γ-ray energy is E, the energy spectrum spread a can be expressed by equation (1).σ=E/√N   (1)
Therefore, when the value N becomes smaller, the value σ increases to spread the energy spectrum. When the energy spectrum is spread, the correlation between the incident γ-ray energy and the signal generated by a DOI detector is impaired. As a result, this makes it difficult to accurately measure the incident γ-ray energy.
If incident γ-ray energy measurements cannot be accurately made, it is difficult to remove scattered radiation contained in incident γ-rays. In PET, the signal output from a radiation detector is passed through an energy filter for scattered radiation removal so as to detect only γ-rays that have a specific energy level or higher. However, if the energy spectrum is spread and, for example, the radiation detector signal output generated by 511 keV γ-rays cannot be differentiated from the radiation detector signal output generated by 300 keV γ-rays, it is necessary to use an energy filter rated at 300 keV or lower. In this instance, 300 keV or higher scattered radiation is also measured so that the amount of noise increases. This can cause PET image deterioration.
SPECT is a method for administering a radiopharmaceutical (hereinafter referred to as a “SPECT pharmaceutical”) containing single-photon-emitting nuclides (99Tc, 67Ga, 201Tl, etc.), which are radionuclides, and glucose or other substance that gathers around specific tumors or molecules, to a medical examinee, and detecting a γ-ray emission from radionuclides with a radiation detector. The energy of γ-ray emission from single-photon-emitting nuclides, which are frequently used for SPECT examination, is approximately several hundred keV. In SPECT, a single γ-ray is emitted so that the angle of γ-ray incidence upon a radiation detector cannot be determined. Therefore, a collimator is used to obtain angular information by detecting only the γ-radiation incident at a specific angle. The SPECT is an examination method for detecting γ-rays generated within a medical examinee's body due to the SPECT pharmaceutical for the purpose of identifying the locations where the SPECT pharmaceutical is heavily consumed. The data obtained is converted to individual voxel data by the filtered back projection or like method as is the case with PET. It should be noted that transmission images may also be generated in SPECT. The half-life period of 99Tc, 67Ga, and 201Tl, which are used for SPECT, is longer than that of PET radionuclides and from 6 hours to 3 days.
X-ray CT (computed tomography) is a method for exposing a medical examinee to radiation emitted from a radiation source and imaging the conformation within the examinee's body in accordance with radiation transmittance in the examinee's body. The intensity of X-rays passing through the examinee's body, which is measured with a radiation detector, is used to determine the coefficient of linear attenuation within the examinee's body between the X-ray source and radiation detector. The determined linear attenuation coefficient is used to determine the linear attenuation coefficient of each voxel by the aforementioned filtered back projection method. The resulting value is then converted to a CT value.
A flat panel detector is a flat radiation detector for use in digital X-ray examination, which is a digital version of conventional X-ray examination. Being equipped with such a flat radiation detector instead of a conventional X-ray film, a fiat panel detector imaging device detects X-rays passing through a medical examinee's body, handles the information about attenuation within the examinee's body as digital information, and displays the digital information on a monitor. The flat panel detector imaging device does not require the use of X-ray film or other media and displays an image immediately after image exposure.
For maintenance of examination accuracy, all these radiological imaging apparatuses require their radiation detectors to be subjected to detection efficiency calibration at periodic intervals of, for instance, three months. The radiation detector's detection efficiency deteriorates with time. However, the deterioration characteristic varies from one radiation detector to another. It is therefore necessary to determine the detection efficiency of each radiation detector on a periodic basis. In PET or SPECT examination in which the number of photons incident on each radiation detector is measured, correct measurement cannot be made if the detection efficiency varies from one radiation detector to another. Therefore, the detection efficiency of each radiation detector is determined beforehand, and the value of each radiation detector is multiplied by the reciprocal of the determined detection efficiency value in order to compensate for image deterioration resulting from the detection efficiency variation of radiation detectors. In X-ray CT or flat panel detector examination, on the other hand, the X-ray intensity is detected by radiation detectors; however, intensity measurements need to be corrected if the detection efficiency varies.
As explained above, the use of radiological imaging apparatuses entails an enormous amount of time and labor because they require their radiation detectors to be checked for detection efficiency variation in order to maintain examination accuracy.